The present invention relates to a process for producing molten metal (which term includes metal alloys), in particular although by no means exclusively iron, from a metalliferous feed material, such as ores, partly reduced ores and metal-containing waste streams, in a metallurgical vessel containing a molten bath.
The present invention relates particularly to a molten metal bath-based direct smelting process for producing molten metal from a metalliferous feed material.
A process that produces molten metal directly from a metalliferous feed material is generally referred to as a xe2x80x9cdirect smelting processxe2x80x9d.
One known direct smelting process, which is generally referred to as the Romelt process, is based on the use of a large volume, highly agitated slag bath as the medium for smelting top-charged metal oxides to metal and for post-combusting gaseous reaction products and transferring the heat as required to continue smelting metal oxides. The Romelt process includes injection of oxygen-enriched air or oxygen into the slag via a lower row of tuyeres to provide slag agitation and injection of oxygen into the slag via an upper row of tuyeres to promote post-combustion. In the Romelt process the metal layer is not an important reaction medium.
Another known group of direct smelting processes that is slag-based is generally described as xe2x80x9cdeep slagxe2x80x9d processes. These processes, such as DIOS and AISI processes, are based on forming a deep layer of slag with 3 regions, namely: an upper region for post-combusting reaction gases with injected oxygen; a lower region for smelting metal oxides to metal; and an intermediate region which separates the upper and lower regions. As with the Romelt process, the metal layer below the slag layer is not an important reaction medium.
Another known direct smelting process, which relies on a molten metal layer as a reaction medium and is generally referred to as the HIsmelt process, is described in International application PCT/AU96/00197 (WO 96/31627) in the name of the applicant.
The HIsmelt process as described in the International application comprises:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in a vessel;
(b) injecting into the bath:
(i) a metalliferous feed material, typically metal oxides; and
(ii) a solid carbonaceous material, typically coal, which acts as a reductant of the metal oxides and a source of energy; and
(c) smelting the metalliferous feed material to metal in the metal layer.
The HIsmelt process also comprises post-combusting reaction gases, such as CO and H2, released from the bath in the space above the bath with oxygen-containing gas and transferring the heat generated by the post-combustion to the bath to contribute to the thermal energy required to smelt the metalliferous feed material.
The HIsmelt process also comprises forming a transition zone above the nominal quiescent surface of the bath in which there are ascending and thereafter descending droplets or splashes or streams of molten metal and/or slag which provide an effective medium to transfer to the bath the thermal energy generated by post-combusting reaction gases above the bath.
An object of the present invention is to provide an improved direct smelting process.
According to the present invention there is provided a process for direct smelting a metalliferous feed material which includes the steps of:
(a) supplying metalliferous feed material and coal to a pre-reduction vessel;
(b) partially reducing metalliferous feed material and substantially devolatilising coal in the pre-reduction vessel and producing a partially reduced metalliferous feed material and char;
(c) supplying the partially reduced metalliferous feed material and char produced in step (b) to a direct smelting vessel;
(d) supplying pre-heated air or oxygen-enriched air to the direct smelting vessel; and
(e) direct smelting the partially reduced metalliferous feed material to molten metal in the direct smelting vessel using the char as a source of energy and as a reductant and post-combusting reaction gas produced in the direct smelting process with the pre-heated air or oxygen-enriched air to a post-combustion level of greater than 70% to generate heat required for the direct smelting reactions and to maintain the metal in a molten state.
The process is particularly, although by no means exclusively, relevant to medium and high volatile coals. Medium volatile coals are understood herein to mean coals containing 20-30 wt % volatiles. High volatile coals are understood herein to mean coals containing in excess of 30 wt % volatiles.
In the case of medium and high volatile coals, the basis of the present invention is the realisation that substantial devolatilisation of these coal types prior to introducing the coal into a direct smelting vessel makes it possible to operate economically a direct smelting process at post-combustion levels of 70% or more using heated air or oxygen-enriched air as the oxygen-containing gas for post-combustion.
Preferably step (b) produces partially reduced metalliferous feed material having a pre-reduction degree of less than 65%.
Preferably, the oxygen concentration in the oxygen-enriched air is less than 50 vol. percent.
The term xe2x80x9csubstantially devolatilisingxe2x80x9d means removal of at least 70 wt. percent of the volatiles from coal.
The term xe2x80x9cpost-combustionxe2x80x9d is defined as:             [              CO        2            ]        +          [                        H          2                ⁢        O            ]                  [              CO        2            ]        +          [                        H          2                ⁢        O            ]        +          [      CO      ]        +          [              H        2            ]      
where:
[CO2]=volume % of CO2 in off-gas;
[H2O]=volume % of H2O in off-gas;
[CO]=volume % of CO in off-gas; and
[H2]=volume % of H2 in off-gas.
The term xe2x80x9coff-gasxe2x80x9d is defined herein as gas generated by smelting reactions and post-combustion and prior to optional addition of any further carbonaceous feed material such as natural gas into that gas.
Preferably the process includes pre-heating air or oxygen-enriched air for step (d) to a temperature in the range of 800-1400xc2x0 C. and thereafter supplying the pre-heated air or oxygen-enriched air to the direct smelting vessel in step (d).
More preferably the temperature is in the range of 1000-1250xc2x0 C.
Preferably the process includes using off-gas discharged from the direct smelting vessel as a source of energy for pre-heating air or oxygen-enriched air prior to supplying the heated air or oxygen-enriched air to the direct smelting vessel in step (d).
Preferably the process includes cooling the off-gas discharged from the direct smelting vessel prior to using the off-gas as the energy source.
Preferably the process includes using part of the off-gas discharged from the pre-reduction vessel as a source of energy for pre-heating air or oxygen-enriched air prior to supplying the heated air or oxygen-enriched air to the direct smelting vessel in step (d).
Preferably the process includes pre-heating the air or oxygen-enriched air in one or more than one hot blast stove.
Preferably the process includes pre-heating the metalliferous feed material prior to step (a) of supplying the metalliferous feed material to the pre-reduction vessel.
Preferably the process includes pre-heating the metalliferous feed material using off-gas discharged from the pre-reduction vessel.
Preferably the pre-reduction vessel is a fluidised bed.
More preferably the process includes recycling off-gas discharged from the fluidised bed back to the fluidised bed.
Preferably the process includes recycling at least 70% by volume of the off-gas discharged from the fluidised bed back to the fluidised bed.
The term xe2x80x9cfluidised bedxe2x80x9d is understood herein to include both bubbling and circulating types. Combination bubbling and circulating are also included.
The term xe2x80x9cmetalliferous feed materialxe2x80x9d is understood herein to mean any metalliferous feed material, which includes metal oxides, such as ores, partly reduced ores and metal-containing waste streams.
Step (e) may be any suitable direct smelting process.
Preferably step (e) includes direct smelting the partially reduced metalliferous feed material in accordance with the HIsmelt process which includes:
(a) forming a molten bath having a metal layer and a slag layer on the metal layer in the direct smelting vessel;
(b) injecting the metalliferous feed material and the char into the metal layer via a plurality of lances/tuyeres;
(c) smelting the metalliferous feed material to molten metal substantially in the metal layer;
(d) causing molten metal and slag to be projected as splashes, droplets, and streams into a space above a nominal quiescent surface of the molten bath and forming a transition zone; and
(e) injecting the pre-heated air or oxygen-enriched air into the direct smelting vessel via one or more than one lance/tuyere and post-combusting reaction gases released from the molten bath, whereby the ascending and thereafter descending splashes, droplets, and streams of molten metal and slag in the transition zone facilitate heat transfer to the molten bath, and whereby the transition zone minimises heat loss from the vessel via the side wall in contact with the transition zone.
The term xe2x80x9cmetal layerxe2x80x9d is understood herein to mean a region or zone that is predominantly metal. Specifically, the term covers a region or zone that includes a dispersion of molten slag in a metal continuous volume.
The term xe2x80x9cquiescent surfacexe2x80x9d in the context of the molten bath is understood herein to mean the surface of the molten bath under process conditions in which there is no gas/solids injection and therefore no bath agitation.